The present invention relates to scintillation detectors. The present invention also relates to a method of operation of a scintillation detector. Whilst the principles of the present invention are applicable to scintillation detectors for various applications, an illustrative embodiment of the invention relates to an assembly including a scintillation crystal for use in a gamma camera for nuclear medicine.
For ease of understanding the present invention and its background it will be illustratively described in relation to a gamma camera.
A gamma camera, also known as an Anger camera, is used to produce a medical diagnostic image. A radioactive substance emitting gamma radiation is injected into a subject to be imaged. The gamma radiation is sensed by a scintillation crystal combined with photomultiplier tubes. The tubes convert scintillation events of visible light into electrical signals which are processed to produce the image.
For a better understanding of the background to the present invention, reference will now be made to FIGS. 1 and 2 which are partial sectional views of parts of known gamma cameras, each section being taken in a plane through an axis (A) of symmetry of the part.
Referring to FIG. 1 a scintillation crystal 1, of sodium iodide is hermetically sealed in a protective enclosure 2 because the crystal is very hygroscopic and fragile. The enclosure 2 comprises a wall of light transmissive and electrically insulative material 3, and a metal cap 4, typically an aluminium cap, which is sealed to the wall 3 by e.g. epoxy resin 6.
The wall 3, in this example is of glass, preferably Pyrex (trade name) glass or the like. Pyrex is a heat resistant borosilicate glass.
An array of photomultiplier tubes 5 are supported by the glass wall. Optical couplant (not shown) is provided between the faces of the tubes 5 and the glass wall 3. Light from scintillation events within the crystal 1 is coupled to the array of tubes 5 by the glass wall.
The enclosure 2 and the tubes 5 are supported by a support structure 7 also known as a `tub`. The tub 7, in addition to supporting the enclosure 2 and tubes 5, shields them from stray radiation and from substantially all light. Typically the tub 7 is of a Lead/Aluminium alloy.
In an alternative structure shown in FIG. 2, an enclosure 2' comprises a steel frame 8 to which the glass wall 3 and aluminium cap 4 are sealed by sealant 6. The cap may also be fixed to the frame e.g. by metal screws. The steel frame makes the enclosure 2' more robust and provides a means for lifting the enclosure in and out of the tub 7 by e.g. fixing lifting bolts (not shown) to the frame 8. The frame 8 is supported by the tub 7.
In the cameras of FIGS. 1 and 2, the photomultiplier tubes are operated with their cathodes at a negative high potential (--HV) relative to their anodes which are grounded. The tub 7, cap 4, and the frame 8, are grounded for safety.
An unexpected problem which has arisen with the cameras of FIGS. 1 and 2 is the occurrence of unwanted scintillations detected by the tubes 5 as background to true scintillation events. As far as is known at present by the inventors, nothing has previously been written about this phenomenon.